Carbon is a platform element in a wide variety of industries and has a vast number of chemical, material, and fuel uses. Carbon is a good fuel to produce energy, including electricity. Carbon also has tremendous chemical value for various commodities and advanced materials, including metals, metal alloys, composites, carbon fibers, electrodes, and catalyst supports. For metal making, carbon is useful as a reactant, for reducing metal oxides to metals during processing; as a fuel, to provide heat for processing; and as a component of the final metal alloy. Carbon is a very important element in steel since it allows steel to be hardened by heat treatment.
Carbon-based reagents can be produced, in principle, from virtually any material containing carbon. Carbonaceous materials commonly include fossil resources such as natural gas, petroleum, coal, and lignite; and renewable resources such as lignocellulosic biomass and various carbon-rich waste materials.
Biomass is a term used to describe any biologically produced matter, or biogenic matter. The chemical energy contained in biomass is derived from solar energy using the natural process of photosynthesis. This is the process by which plants take in carbon dioxide and water from their surroundings and, using energy from sunlight, convert them into sugars, starches, cellulose, hemicellulose, and lignin. Of all the renewable energy sources, biomass is unique in that it is, effectively, stored solar energy. Furthermore, biomass is the only renewable source of carbon.
By utilizing biogenic carbon for fuel, CO2 emissions associated with the combustion do not contribute to net life-cycle carbon emissions because carbon is recycled to grow more biomass. Also, use of biogenic carbon as a fuel will typically cause lower emissions of sulfur dioxide and mercury, compared to use of coal or other solid fossil fuels for energy production.
For chemical and material applications in which the carbon will not be immediately combusted, by utilizing biogenic carbon, the carbon can be effectively sequestered for long periods of time (e.g., when carbon is added to steel for permanent structures). In this way, the net carbon emissions are actually negative—CO2 from the atmosphere is used to grow biogenic feedstocks and then the carbon is sequestered in biogenic products.
Converting biomass to high-carbon reagents, however, poses both technical as well as economic challenges arising from feedstock variations, operational difficulties, and capital intensity. There exist a variety of conversion technologies to turn biomass feedstocks into high-carbon materials. Most of the known conversion technologies utilize some form of pyrolysis.
Pyrolysis is a process for thermal conversion of solid materials in the complete absence of oxidizing agent (air or oxygen), or with such limited supply that oxidation does not occur to any appreciable extent. Depending on process conditions and additives, biomass pyrolysis can be adjusted to produce widely varying amounts of gas, liquid, and solid. Lower process temperatures and longer vapor residence times favor the production of solids. High temperatures and longer residence times increase the biomass conversion to syngas, while moderate temperatures and short vapor residence times are generally optimum for producing liquids. Recently, there has been much attention devoted to pyrolysis and related processes for converting biomass into high-quality syngas and/or to liquids as precursors to liquid fuels.
On the other hand, there has been less focus on improving pyrolysis processes specifically for optimizing yield and quality of the solids as high-carbon reagents. Historically, slow pyrolysis of wood has been performed in large piles, in a simple batch process, with no emissions control. Traditional charcoal-making technologies are energy-inefficient as well as highly polluting. Clearly, there are economic and practical challenges to scaling up such a process for continuous commercial-scale production of high-quality carbon, while managing the energy balance and controlling emissions.